Add attributes to Pub/Sub tracing

- [ ] Add the following attribute: `gcp.project_id`
- [ ] Add the server address and port for the [gRPC spans](https://github.com/googleapis/google-cloud-cpp/blob/main/google%2Fcloud%2Finternal%2Fgrpc_opentelemetry.cc#L79-L83)

Start with the gRPC OpenTelemetry implementation in google/cloud/internal/grpc_opentelemetry.cc, especially the referenced span-attribute section, and trace how Pub/Sub spans are created. Confirm where the project ID and server address and port belong, then verify that the resulting Pub/Sub gRPC spans expose all requested attributes.

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